Fastener, power tool for use with same and method tightening the fastener

ABSTRACT

Attainment of a predetermined stress in a bolt (10) which is being tightened is indicated by interference with the freedom of a pin (15) to rotate relative to the bolt. The pin extends along a bore (14) from an end of the bolt and the bore lies in that part of the bolt which presents the minimum diameter of the bolt. The invention is especially useful for detecting when the stress to which the bolt is subjected exceeds the elastic limit of the bolt, because the radial contraction of the bore which then occurs is large, relative to the contraction which occurs during elastic deformation of the bolt.

DESCRIPTION OF INVENTION

The present invention is concerned with screw-threaded fasteners,examples of which include bolts and studs. It is a common requirementfor bolts and studs to be tightened to a predetermined load. Variousproposals have been made to achieve this. These includes the use oftorque wrenches for tightening and the provision, in a fastener, ofmeans for indicating when a predetermined load has been achieved.

Known means provided in threaded fasteners for indicating when apredetermined load has been achieved are responsive to elongation of thefastener.

According to a first aspect of the present invention, there is provideda method of tightening a threaded fastener wherein a predeterminedchange in a diameter or other transverse dimension of the fastener isindicated or detected. Generally, threaded fasteners are elongated andhave circular transverse cross sections. However, the unthreaded part ofa bolt may have a transverse cross section other than circular.

The fastener preferably has indicating means so arranged as to be freeto move relative to the fastener, when the fastener is subjected tostress of one value, and to be restrained against movement relative tothe fastener when the fastener is subjected to a stress of asubstantially different value. However, it is also within the scope ofthe invention for the predetermined change in a diameter of the fastenerto affect the range of possible movement of the indicating meansrelative to the fastener.

The changes in diameter, in particular contraction, of a fastener whichoccur when the fastener is subjected to tension beyond its elastic limitare more pronounced than are the changes in diameter which occur whenthe fastener is stressed within its elastic limit. Accordingly, theinvention is more readily applied to tightening of a fastener to adegree such that the elastic limit is exceeded.

According to a second aspect of the invention, there is provided athreaded fastener having indicating means responsive to a change in adiameter or other transverse dimension of the fastener for indicatingwhen a selected value of stress has been attained in the fastener.

The preferred fastener is hollow, the indicating means lying at leastpartly inside the fastener. However, a fastener with indicating meanswhich overlies a part of a circumferential surface of the fastener isalso within the scope of the invention.

According to a third aspect of the invention, there is provided a powertool for tightening a screw-threaded fastener, the tool having controlmeans for controlling operation of the tool and signalling means forco-operating with the indicating means of a fastener according to thesecond aspect of the invention and with the control means to transmit tothe control means a signal indicating that said selected value of stresshas been attained in the fastener.

Examples of fasteners embodying the second aspect of the invention,which are used in a method according to the first aspect and an exampleof a power tool in accordance with the third aspect of the inventionwill now be described, with reference to the accompanying drawing,wherein:

FIG. 1 shows a cross-section of a bolt in a plane containing alongitudinal axis o the bolt,

FIG. 2 shows a similar view of a stud,

FIG. 3 shows a similar representation of the combination of a furtherbolt and a power tool for turning the bolt; and

FIG. 4 shows a further bolt partly in diammetral cross-section.

The bolt 10 shown in FIG. 1 comprises a shank 11 having on one endportion thereof a screw-thread 12 and a head 13 at the opposite end ofthe shank. The threaded end portion of the shank is hollow and defines acylindrical bore 14 which extends longitudinally of the bolt and, in theexample illustrated, is co-axial with the thread 12.

Indicating means 15 is disposed in the bore 14. The indicating means isin the form of a pin having a head 16 which, as shown in FIG. 1, may bereceived in a recess in the end of the shank 11. The pin 15 includes acylindrical shank portion 17 which lies radially inwardly of that partof the thread 12 which is near to the unthreaded part of the shank 11.An intermediate part of the pin which extends between the cylindricalportion 17 and the head 16 may have the same diameter as or a smallerdiameter than that of the portion 17. It will be noted that there is aclearance space between the end of the pin and the blind end of the bore14.

The diameter of the cylindrical portion 17 is such that, when the bolt10 is unstressed, the portion 17 is a close fit in the bore 14 but thepin 15 is free to turn about the axis of the bore 14 relative to thebolt. Turning of the pin can be effected by means of a hand tool, forexample an allen key introduced into an appropriate socket in the head16 of the pin.

When the bolt 10 is tightened into a nut (not shown) on the thread 12,with an assembly (also not shown) which is to be held by the bolt beingtrapped between the head 13 of the bolt and the nut, the shank 11 of thebolt is subjected to tensile stress. This causes elongation of the boltand consequent radial contraction of the shank and of the bore 14.Provided the stress is below the elastic limit of the bolt, the radialcontraction is slight and occurs along at least the threaded part of theshank 11.

If the stress is increased, by tightening of the bolt, to the elasticlimit of the bolt, further tightening causes more pronounced contractionof the shank 11 and of the bore 14 but this contraction occurs mianly inthat part of the shank which is immediately adjacent to a face of thenut presented towards the head 13. This part of the shank contains thecylindrical portion 17 and significant contraction of the bore 14results in the shank gripping the portion 17 firmly so that rotation ofthe pin 15 relative to the bolt 10 is restrained. There is thus providedan indication that the elastic limit of the bolt has been reached.

Alternatively, the respective diameters of the bore 14 and portion 17may be such that the pin 15 is free to rotate relative to the bolt 10when the latter is unstressed but is restrained against rotation, bygripping of the portion 17 within the bore 14, when a predeterminedstress which is less than the elastic limit of the bolt is attained. Itwill be appreciated that, because the contraction which occurs when thebolt is stressed elastically is considerably less than the contractionwhich occurs when the bolt is stressed beyond its elastic limit, therespective diameters of the bore and cylindrical portion 17 must becontrolled within smaller tolerances if the pin 15 is required to givean indication of the attainment of a stress which is less than theelastic limit. Furthermore, the attainment of the elastic limit can beindicated with somewhat greater accuracy than can the attainment of somelower, predetermined stress.

In FIG. 2, there is illustrated a stud which differs from the bolt ofFIG. 1 in htat the stud of FIG. 2 is not hellow. In FIG. 2, partscorresponding to those hereinbefore described with reference to FIG. 1are indicated by a like reference numeral with the prefix I and thepreceding description is deemed to apply, except for the differencesmentioned.

The indicating means of the stud shown in FIG. 2 is a ring 115 whichoverlies the peripheral surface of a part 118 of the shank 111 of thestud. The part 118 has the form of a neck, having a diameter which isnot greater than, and is preferably somewhat less than the root diameterof the thread 112. Preferably, the shank 111 has the minimum diameterover a substantial part of its length, the neck presenting a cylindricalsurface between shoulders at opposite ends of that surface.Alternatively, the diameter of the shank may have a minimum value over arelatively short part of the length of the shank, the shank defining arelatively narrow groove at the neck.

When the stud 110 is unstressed, the ring 115 is an interference fit onthe neck 118 and is thus restrained against rotation relative to thestud. When the stud is tightened, it is subjected to tensile stress, thelength of the shank 111 is increased somewhat and the diameter of theshank is decreased slightly. The ring 115 is, however, not subjected toany significant tensile stress and its diameter does not change. Whenthe stress to which the stud 110 is subjected exceeds the elastic limitof the stud, the material yields in the neck 118 and there is pronouncedradial contraction of the neck relative to the other parts of the studand relative to the ring 115. In consequence of this, the mutualgripping relation of the ring and the neck is terminated and the ring isfree to turn on the neck. Provided the ring is accessible, the freedomof the ring to turn can be checked during tightening of the stud and anindication thereby obtained when the elastic limit of the stud has beenattained.

It will be understood that the stud of FIG. 2 may be provided withindicating means in the form of a pin, rather than in the form of aring. In this case, the pin would be placed in a bore extending alongthe axis of the stud from one end thereof past the neck 118 wouldconveniently be positioned somewhat nearer to one end of the stud thanto the other end. Alternatively, the provision of a neck may be avoided,the pin indicating a change in the diameter of the bore at a positionalong the stud which corresponds to a part of one of the threads on thestud.

In FIG. 3, there is illustrated a bolt 210 and certain parts of a powertool 220 used for tightening the bolt. Parts of the bolt 210 whichcorrespond to parts of the bolt 10 are indicated by like referencenumerals with the prefix 2 and the preceding description is deemed toapply, except for the differences hereinafter mentioned.

The power tool 220 has driving means 221 which may be of known form anddefines a polygonal recess for receiving the head 213 of the bolt. Amotor, for example an air motor, (not shown) is provided for applyingtorque to the driving means 221. Control means 223 is provided forcontrolling operation of the motor. There is operatively associated withthe control means signalling means 224 for transmitting to the controlmeans a signal indicating that a predetermined change in a diameter ofthe bolt 210 has occurred. In the example illustrated in FIG. 3, thesignalling means has the form of a rod which extends from the controlmeans through the interior of the driving means 221 and which has afree-end portion with a shape and size complimentary to the shape andsize of the recess in the head 216 of the pin 215. The control means 223is arranged to apply torque to the rod 224 and to permit operation ofthe motor of the tool 220 whilst the rod rotates. The rod is free torotate with the pin 215 when the bolt 210 is unstressed.

As the bolt 210 is tightened, the bore 214 contracts and a predeterminedchange in the diameter of the bore establishes a mutually grippingrelation between the pin and the bolt, thus impeding rotation of the pinand of the signalling means 224. The control means 223 responds to thisby terminating operation of the motor of the tool 220. The control meansmay comprise a known electric transducer which provides an electricaloutput signal indicating whether the signallig means 224 is turning ornot. This output signal may be used to control operation of the motor.

The relative dimensions of the bore 214 and pin 215 may be selected toprovide that tightening of the bolt will be terminated when the bolt hasbeen subjected to a stress which is below the elastic limit of the bolt.Alternativeely, the pin 215 may be free to rotate until the elasticlimit of the bolt is exceeded.

In FIG. 3, the tool 220 is illustrated with the driving means 221engaged with the head 213 of the bolt for driving same. Accordingly, thebore 214 extends into the bolt from the head end thereof and the head216 of the pin is accessible within a recess in the head 213 of thebolt. Alternatively, the tool may be engaged with a nut 225 on thethreaded part of the bolt for driving the nut along the thread of thebolt. In this case, the bore 214 would extend into the bolt from the endthereof opposite to the head and the head 216 of the pin would beaccessible in a recess provided in the shank 211 of the bolt. In a casewhere the bore 214 lies in a threaded part of the bolt, the minimumdiameter of the bolt may be presented by that threaded part. In the caseillustrated in FIG. 3, where the bore 214 lies in the head and anadjacent part of the bolt, the minimum diameter of the bolt is presentedby a part of the shank which contains the bore 214 and the pin 215 andthis part may constitute an unthreaded neck 218.

Whilst it is necessary for a mutually gripping relation to beestablished between the pin 215 and the bolt 210 in the neck 218, duringtightening of the bolt, it is not necessary for such relation to beestablished at other positions along the length of the bolt. Thus,either the pin 215 or the bore 214 may be of stepped form, there being asubstantial radial clearance between the pin and the surface of thatpart of the bore 214 which lies in the head 213.

In a further alternative arrangement, either the bore or the pin is oftapered form. This arrangement also provides substantial clearancebetween the pin and the surface of the bore in regions where radialcontraction of the bore will be relatively small, but enables a mutuallyinterfering relation to be established between the pin and the boundarysurface of the bore in a region where radial contraction of the bolt issomewhat greater. Furthermore, this arrangement enables a predeterminedcontraction of the bore to be indicated either by restraint of rotationof the pin relative to the bolt or by limitation of the extent to whichthe pin can be introduced into the bore. Thus, the distance which thepin can travel along the bore may be checked intemittently, duringtightening, until that travel is limited to a predetermined distance,corresponding to a predetermined radial contraction of the bore.

In FIG. 4, there is shown a bolt and nut. Parts of the boltcorresponding to parts of the bolt described with reference to FIG. 1are identified by like reference numerals with the prefix 3 and thepreceding description is deemed to apply, except for the differenceshereinafter mentioned.

The pin 315 lies mainly in the bore 314 but includes amn end portionprotruding from the bore. There is also disposed within the bore acoiled compression spring 320 which acts between the blind end of thebore and the pin 315 and opposes movement of the pin along the bore to aposition in which the pin is contained entirely within the bore. Thespring may be attached to the pin, so that the spring limits movement ofthe pin in a direction outwardly of the bore. At least one turn of thespring adjacent to the blind end of the bore may be an interference fitin the bore.

When the bolt of FIG. 4 is unstressed, the pin 315 is free to slidealong the bore and can easily be depressed, by finger pressure, into thebore. When the bolt has been subjected to a predetermined tensilestress, the pin is gripped by the walls of the bore 314 so that slidingof the pin along the bore is restrained. The application of fingerpressure to the end of the pin fails to move the pin along the bore andthis indicates that a predetermined stress has been attained.

It will be noted that the pin 315 lies in the threaded part of the bolt.The example of pin illustrated in FIG. 4 is of uniform diameter alongits entire length. Alternatively, the pin may be of stepped form, an endportion of the pin nearest to the blind end of the bore having a largerdiameter so that this end portion of the pin can be gripped by the bolt,when subjected to the predetermined stress, whilst the remainder of thepin is of smaller diameter and is not gripped.

To avoid thermal expansion and contraction affecting the degree ofaccuracy with which the tension in a bolt can be indicated by the pin,the bolt and the pin are preferably formed of the same material or ofmaterials which have the same co-efficient of thermal expansion.Generally, the bolt and pin will be formed of steel.

The diameter of the bore 14 is typically within the range one millimeterto six millimeter. Preferably, this diameter is within the range twomillimeter to three millimeter. In a case where a part only of the shankof a bolt is threaded, the length of the bore preferably exceeds thelength of the thread but does not exceed twice the length of the thread.Particularly in a case where the pin lies entirely inside the fastener,the length of the pin is somewhat less than that of the bore. Whilst weprefer the pin to lie entirely inside the fastener, in order that thefastener shall protect the pin against mechanical damage, the head orother end portion of the pin may lie outside the fastener. In this case,the provision of a recess in the end of the pin may be avoided. The pinmay be gripped externally by the fingers of a user or by a tool, inorder to apply torque to the pin.

The features disclosed in the foregoing description, or the accompanyingdrawings, expressed in their specific forms or in terms of a means forperforming the disclosed function, or a method or process for attainingthe disclosed result, as appropriate, may, separately or any combinationof such features, be utilised for realising the invention in diverseforms thereof.

I claim:
 1. In a hollow fastener which has a transverse cross section and which includes a portion where the area of the transverse cross-section of the fastener has a minimum value, which portion is subjected to tensile stress when the fastener is in use, indicating means which comprises a stem, wherein the stem lies inside the fastener and is free to turn relative to the fastener when the fastener is unstressed and wherein a part of the stem which lies within said portion of the fastener is gripped inside the fastener by said portion of the fastener to restrain turning of the stem relative to the fastener when the fastener is subjected to a predetermined tensile stress.
 2. A fastener according to claim 1 wherein the stem has a larger diameter portion and a smaller diameter portion and wherein the larger diameter portion of the stem lies inside said portion of the fastener.
 3. A fastener according to claim 2 wherein the diameter of the larger diameter portion of the stem does not exceed three millimeter.
 4. A fastener according to claim 1 wherein the internal diameter of the fastener vaires along the length of the fastener.
 5. A fastener according to claim 1 wherein said indicating means further comprises a head at one end of the stem and wherein the head overlies a surface of the fastener.
 6. A power tool for tightening a screw-threaded fastener, the tool having control means for controlling operation of the tool and signalling means for co-operating with the indicating means of a fastener according to claim 1 and with the control means to transmit to the control means a signal indicating that said selected value of stress has been attained in the fastener. 